White matter damage is a central event in the pathophysiology of diverse CNS disorders, including stroke and vascular dementia. But most cerebrovascular studies still focus on gray matter. In our previous grant cycle, we developed the novel concept of the oligovascular niche, wherein cerebral endothelial cells support oligodendrocyte precursor cells (OPCs). However, near the end of our grant period, we discovered that this crosstalk may be bi-directional. OPCs can in turn support endothelial function including blood-brain barrier (BBB) integrity. Importantly, this bi-directional interactio between OPCs and cerebral endothelium does not stand in isolation. Our pilot data suggest that crosstalk between cerebral endothelium and OPCs may be modulated by other cells such as astrocytes. Therefore, this renewal grant will expand our oligovascular niche concept by testing the overall hypothesis that OPC-derived trophic factors regulate BBB function in white matter, and astrocytes are critical for regulating/modulating these bi-directional processes of oligovascular signaling. We will extend our pilot data and attempt to validate our hypothesis with three specific aims: Aim 1 will define the mechanism for how OPCs support endothelial permeability in vitro and BBB in vivo, Aim 2 will assess the role of astrocytes in regulating OPC-endothelial interactions in vitro, and Aim 3 will validate the role of astrocytes for regulating OP-BBB interactions in vivo. These experiments should extend our original idea of the oligovascular niche into a more general gliovascular unit, wherein astrocytes regulate crosstalk between OPCs and endothelium. Dissecting cell-cell signaling pathways in this conceptual framework may lead us to new therapeutic treatments for white matter-related diseases in the CNS.